Seminar 6 2010 Discussion Page


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LENScience Senior Biology Seminar Series 2010 Challenge 2
Challenge 3 Harnessing Biodiversity
Seminar 6 2010



Harnessing Biodiversity Discussion Page: Challenge 1

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SouthCity High School: Student BC -  25th April 2pm

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Michal Denny 16th August

Hi everyone, remember to check out the useful websites on the seminar main page as part of your preparation for answering these questions.


Lack of appropriate food resources can affect the survival and effectiveness of natural enemies. One effect of insufficient resources is on sex ratios of the adults. Explain how a sex ratio that is biased towards males can impact on the ability of a parasitic wasp to control a pest.


'Riccarton High School': Student S.N - 30th August, 8.00pm

A sex ratio that is biased towards males suggests that males may be more successful competitors for food than females when resources are insufficient - perhaps due to a physical advantage in being stronger than female wasps or not having to use energy in producing eggs and finding hosts for the eggs to deposit into. The parasitic wasp controls pests by 'injecting' the eggs into the host's body - when the eggs are ready to hatch, the larvae eat their way out of the host's body, effectively killing the host. As it is the female wasps that carry the eggs, it is therefore the female wasps which actually control the pest. As the sex ratio is biased towards males when there is an insufficient food supply, there are less female parasitic wasps to lay eggs in their hosts, resulting in a fall in the effectiveness of their control of the pest. In addition to this initial loss of control, the decrease in the amount of eggs laid in the hosts results in a decrease in the number of wasps hatching and growing into adults. This means in yet another decrease in the amount of female wasps along with male wasps, which is again affected by the male wasp's advantage in competing for scarce food resources. This would result in a further loss of control of the pest.

Therefore, with a sex ratio which is biased towards males in times of insufficient food source, when resources become scarce, the effectiveness of the parasite wasp to control the pest declines in a downward spiral.


Sharon, Riccarton High School - 1 September 2010


A male biased sex ratio can have both negative and positive effects on its ability to control a pest. As females are responsible for laying eggs that hatch within the pest, fewer females will mean fewer eggs that are laid amongst the hosts, thus hindering its ability as a biological control as fewer numbers of the pest species are being killed. Not only are fewer of the pest species being killed, but there are less eggs hatching and surviving to reproduce themselves. Additionally, if the wasp is being newly introduced to the pest species, fewer founding females will mean a lower chance of establishment of the wasp species as fewer offspring are being produced. This also decreases its ability as a biological control as an established population that can sustain itself is much more effective.
However a male-biased sex ratio could also have positive effects. As there are fewer females for the male wasps to mate with, there is intensified competition between males. This means that more effort is put in to courting females, so females will have a higher chance of encountering males that are better adapted. From this, the resulting offspring will have better genes and are more likely to survive to a reproductive age. Not only are these desired genes passed on, but there would be a higher chance of male mating success, so more offspring overall is produced. Since better adapted offspring with better genes are produced, this has a positive effect on the wasp as a form of biological control.



Michal Denny 2nd September

Well done Sharon and S.N. This question looks straight forward but does require you to think carefully. Both of you have correctly made the link between females and the control. I also like the way that you have extended your thinking to consider the longer term implications of a male biased sex ratio. But do look at Steve's comments below.

Steve Wratten 2nd September

These answers are thoughtful and show good scholarship
Actually, sex ratio in parasitoid wasps is a complicated thing

1. The right nectar leads to the females’ living longer. Most male eggs are laid early in the female’s life, so no access to nectar leads to the her dying earlier –so mainly male eggs are laid.

2. Females store the sperm in their bodies and can ‘decide’ whether or not to fertilise their eggs with it. If the female perceives that her environment is not ‘rich’ ie that it is low in nectar sources, then she and any females she produces may not be ‘fit’ in the sense of passing on their genes to many offspring. She therefore lays a low proportion of female eggs. In that situation, mobile animals such as these parasitoids would probably not hang around but would move away, searching for nectar sources somewhere else


Nathan Young, Horowhenua College, 12 Sept. 4.10pm


A sex ratio higher in males will reduce the efficacy of the Dolichogenidea tasmanica, the parasitic wasp, being able to control a pest.All animals require the food resource for valuable nutrients and energy used for fueling their physical activities but also to reproduce. This is no exception with the parasite wasp. The adult parasitic wasp feeds only on nectar and pollen, and where there is a lack of these resources, the wasps itself, and the ecosystem services it provides will be negatively affected. The herbivorous feeding of the wasp shows it relies primarily on nectar as a source of carbohydrate, and pollen as a source of protein. A strong male bias in the population means that there would be some adaptive advantage of being a male when there is low biodiversity in an area (ie, an agroecosystem). This adaptive significance can probably explain why the ratio of males to females is higher: in a monoculture, there are fewer flowers available for nectar, pollen, and as mating sites for the wasps. Adult males rely on the nectar to provide the energy to aid its survival, while the female needs both for successful reproduction. There is a high energy cost to producing the eggs, as well as a need for protein (from pollen) as a raw material to make the eggs. If flowers are used as mating spots in the wasp, then fewer flowers present will contribute to more virgin females (and as males develop parthenogencially, this increases the male bias). Still, even if the female mates, she may not choose to use her sperm stores to fertilise the eggs if the environment doesn’t contain sufficient food. In a horticultural area, where there are low amounts of pollen and nectar present, females will die earlier than males. The lack of nectar and pollen deduces why there is a strong correlation between low diversity in an agricultural area and the higher ratio of males in the D. tasmanica population there.

The ultimate consequence, for humans, an unequal sex ratio in the parasitic wasp is that we are unable to access the full potential of the wasp as a biological control agent. Humans utilise the wasp as biocontrol against leaf roller larva in grape vineyards in New Zealand and Australia by relying on the female wasps ovipositing their eggs into a host pest caterpillar where the hatched larva can carry out their parasitoid lifestyle within the leaf roller and reduce the numbers of this pest present. Because females are the only sex which produces and lays the eggs, a male biased ratio in the wasps reduces the effectiveness of the wasps as a biocontrol. Although there are many other factors present in a monoculture which affects the overall number of eggs produced by female wasps, the sex ratio is one of the main reasons the net number of eggs laid in the wasp population is impacted on. Also, if there were to be a higher proportion of males in the proportion, then females would be more fiercely competed for. This would either drive males out to find a female partner in another area, or some males will be left to die without having a chance to mate. The mated females still, may not decide to lay the eggs in this area. Because there will be a shortage of nectar and pollen in a place low diversity (usually farmlands) for their female offspring once they reach adulthood, it may be in the female mothers’ interest to move to another area, richer in resources, to lay the eggs. This way, the offspring have a higher chance of survival. So, a male biased sex ratio reduces the number of eggs produced in a population, but also means that these eggs may not be deposited into the pests here as another area could be more plentiful in resources (the primary reason why there are more males here is because there is little nectar and pollen for females to feed on). The ability of D. tasmanica to control a pest is therefore is limited to a low level in a monoculture where there is a higher ratio of males.

However, it is not completely safe to assume that by planting any floral resource will resolve the problem of male bias by providing SNAP. Because the pest species are also insects, their adult individuals will exploit similar food sources as the D. tasmanica. This may constitute an insidious increase of pest numbers as well as parasitoid numbers. So, what is required are carefully planned planting strategies which will enhance the natural enemy’s fitness than the host insect. Further experiments would have to be conducted to find which floral species help the parasitoid to flourish, but minimise the survival of the host species. This can be seen in fig 10 and 11 (sem paper) where Phacelia and Buckwheat benefit the parasitoid more than the host (assuming most of the host eggs are laid in the first four days). 


Michal Denny 4 October

Hi Nathan,a very detailed answer, I thought you explained very well how the male biased sex ratio impacts on the ability of the parasitoid to control the pest. There's just one area that I want to comment on. You wrote A strong male bias in the population means that there would be some adaptive advantage of being a male when there is low biodiversity in an area.Where the adaptive advantage arises is something you need to think carefully about; what occurs is not necessarily because it itself is beneficial - it may be the effect of something else that is more beneficial. In this case the advantage is not in being a male in an area of low biodiversity - the advantage is in not producing females when there is little or no food for them. So producing males is simply the outcome of not producing females.